Industry Segments and Foreign Operations - Sales Revenue by Geographic Region (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	$ 510,947	$ 454,143	$ 1,494,392	$ 1,328,715
Reportable Geographic Regions | United States
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	184,374	177,170	536,122	508,522
Reportable Geographic Regions | Germany
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	53,535	43,782	155,640	132,068
Reportable Geographic Regions | All other countries
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	$ 273,038	$ 233,191	$ 802,630	$ 688,125